Ceramic is widely used as a coating material for hard metal cutting tools since its mechanical properties do not easily deteriorate even at high temperatures. Although there are various ceramic coating materials, Alumina (Al2O3) is particularly well known for its excellent thermal stability, hardness and abrasion resistance among such materials.
Out of various phases of Alumina, α-Al2O3 has excellent thermal resistance and abrasion resistance. Thus, it is preferable to form an α-Al2O3 coating layer on a surface of a cutting tool. However, compared to the formation of a κ-Al2O3 coating layer, it is more difficult to form an α-Al2O3 coating layer on a surface of a cutting tool. Further, an α-Al2O3 coating layer is allowed to form only after a certain coating material such as TiCNO is formed on the substrate of a cutting tool. Transforming the phase of the κ-Al2O3 coating layer, which is molded on the surface of a cutting tool, into an α phase was considered. However, it is difficult to realize this since such a transformation requires initially forming a κ-Al2O3 coating layer on the surface of a cutting tool, melting the cutting tool at a temperature of more than 2000° C. and then instantaneously cooling the cutting tool. Also, the cutting tool melted at a temperature of more than 2000 degrees Celsius can be suddenly cooled. Thus, the K phase has been traditionally used as an alumina coating layer.
A cutting tool with bad surface roughness tends to have a high cutting resistance and easily adheres to a workpiece. Thus, the service life is shortened. A ceramic coating such as α-Al2O3 has bad surface roughness compared to other coating materials for a cutting tool. Thus, such a ceramic coating needs to be surface-treated to obtain an improved surface roughness. Various methods such as blasting, polishing, brushing, etc., are known as a surface treating method for improving surface roughness.
However, a ceramic coating material such as α-Al2O3 is very difficult to surface-treat in the above-mentioned methods due to its hardness. Thus, it takes much time and effort to perform such surface treating operations. Furthermore, it has limits on the improvement of surface roughness.